1. Field of the Invention
This invention relates to an intraocular length measuring instrument for measuring an intraocular length between an intraocular object to be measured and a corneal vertex by finding a corneal vertex position using an optical system utilizing the principle of geometrical optics and finding a position of an intraocular object to be measured using an interference optical system utilizing the principle of physical optics.
2. Brief Description of the Prior Art
Heretofore, an eye axis length measuring instrument is known, in which a light beam from a laser diode LD is irradiated to an eye to be tested and a plane wave reflected from the eye fundus as an intraocular object to be measured is caused to interfere with a spherical wave reflected from the cornea, and a length (eye axis length), as one kind of intraocular length, between the eye fundus and the cornea is measured using the interference signal.
However, this conventional eye axis length measuring instrument has such shortcoming as that when a plane wave reflected from the eye fundus is caused to interfere with a spherical wave reflected from the cornea, a high accuracy of alignment of the measuring instrument is strictly required with respect to an eye to be tested and this is, in a sense, a vital shortcoming for an instrument which is intended to measure a constantly moving eye ball. If the alignment of the measuring instrument is deviated, even in a slightest amount, with respect to the eye, positions of interference fringes are considerably deviated and the number of interference fringes is suddenly increased at a place from where the eye to be measured has been observed till that time. This makes it difficult to tell whether an interference is taken place between the plane wave and the spherical wave.